Aos

[The Radium King]

i understand. what i am wondering is is whether the crankcase needs vacuum or atmospheric pressure.

the pistons are slapping around, fluids are heating, pressure increases in the crankcase above atmospheric pressure. as a result, air leaks past rings, pistons have to work to compress the air in the crankcase, etc. etc.

the 1955 solution is to vent the crankcase to atmosphere so that pressure doesn't build. this venting, however, lets bad gasses out into the environment. to address this, in modern cars the gasses are now vented back into the intake to combust and eventually get scoured by the cats instead (via an aos to remove as much liquid from the gas as possible).

so, you can vent the aos into the intake before the throttle body or after. if you vent after (as per the boxster) you will get vacuum. i see the benefit to vacuum as it will provide some scouring of the system; vent to before the throttle body and you'd get all kinds of deposition of crud.

given that, the requirement for vacuum is not of any benefit to the crank and engine longevity/efficiency. but rather to facilitate introduction of crankcase gasses into the intake. if that is the case, then redirecting the output of the aos to a catchcan at atmospheric pressure is a valid thing to do for engine health (not environmental health, however). dumping it to ground as per TPC isn't, as there is risk that liquid oil can get past the aos and make a big mess.

[JFP in PA]

Originally Posted by The Radium King

i understand. what i am wondering is is whether the crankcase needs vacuum or atmospheric pressure.

the pistons are slapping around, fluids are heating, pressure increases in the crankcase above atmospheric pressure. as a result, air leaks past rings, pistons have to work to compress the air in the crankcase, etc. etc.

the 1955 solution is to vent the crankcase to atmosphere so that pressure doesn't build. this venting, however, lets bad gasses out into the environment. to address this, in modern cars the gasses are now vented back into the intake to combust and eventually get scoured by the cats instead (via an aos to remove as much liquid from the gas as possible).

so, you can vent the aos into the intake before the throttle body or after. if you vent after (as per the boxster) you will get vacuum. i see the benefit to vacuum as it will provide some scouring of the system; vent to before the throttle body and you'd get all kinds of deposition of crud.

given that, the requirement for vacuum is not of any benefit to the crank and engine longevity/efficiency. but rather to facilitate introduction of crankcase gasses into the intake. if that is the case, then redirecting the output of the aos to a catchcan at atmospheric pressure is a valid thing to do for engine health (not environmental health, however). dumping it to ground as per TPC isn't, as there is risk that liquid oil can get past the aos and make a big mess.

Actually, the slight vacuum in the crankcase is critical to the low ring tension design rings creating a seal to the cylinder walls. Eliminate the vacuum below the rings and ring seal deteriorates significantly, blow by becomes rampant, ring “flutter” starts scoring the cylinder walls, and you loose significant HP, not to mention engine longevity. Do not run one of these engines without some level of crankcase evacuation, even if it comes from a supplementary source such as a belt driven vacuum pump, common on high output race engines. On a street engine, it also aids in pollution control, but it cannot be removed without consequences………

[The Radium King]

moment of clarity for me. you don't want to equalise crankcase pressure to atmospheric, you want to equalise to the vacuum on the other side of the piston, otherwise the pressure will try to equalise from crankcase to combustion chamber via the rings.

so what to do in a forced induction situation? checkvalve ont he aos? get your vacuum for a vacuum pump instead?

[JFP in PA]

Originally Posted by The Radium King

moment of clarity for me. you don't want to equalise crankcase pressure to atmospheric, you want to equalise to the vacuum on the other side of the piston, otherwise the pressure will try to equalise from crankcase to combustion chamber via the rings.

so what to do in a forced induction situation? checkvalve ont he aos? get your vacuum for a vacuum pump instead?

You always want to have lower pressure (read a level of vacuum) below the rings in low tension motors than you have above them during the engine's compression cycle on each cylinder; this will aid the low tension rings in obtaining optimum seal to the cylinder walls at the moment they need it most.

How you obtain this condition can vary all over the map. Production cars tend to use manifold vacuum, as the M96 does via the AOS system, and other makes do using a positive crankcase ventilation valve. On a race engine, some use a vacuum pick up in the header collector (slightly behind the point where the pipes come together in the collector, a properly shaped tube mounted on an angle and sealed to the collector housing with create a venturi effect and generate a slight vacuum signal, which can be routed to the crank case with hoses), or using a belt driven vacuum pump (more reliable and tunable) from suppliers like Moroso and others, which also utilize a catch can to trap oil. Either way, you get to the same place: lower parasitic drag engines with higher power output.





Non-atmospheric engines (turbo or blower) tend to go the pump route as manifold vacuum becomes non existent the instant the engine goes into boost. Just routing the AOS line down at the ground or putting in a check valve is obviously not going to provide any crankcase evacuation, and the lack of same is going to cause the rings not to seat properly...............

[The Radium King]

thanks. i was reading on the subi and 944 boards where they go pre-turbo and do similar to your exhaust sketch - use the velocity of the airflow past the vent tube to create vacuum.

[Pat]

FWIW, one of my students had a TPC turbo kit on his 986S. It was super fast and a ton of fun to drive. But every time we'd go to the track he'd dump oil on the track in sustained high-G right turns. He asked TPC about it and they told him to just run lower oil levels. He did. His engine blew up. The end.

[The Radium King]

that's a terrible story. sounds like a failing aos dumping oil; tpc perhaps should have recommended a catchtank instead.

but what do you expect for $10k? i've also read that what they call 'custom turbo' is a made in china garret knock-off; if you want a quality turbo you have to buy their stage II kit. too bad, i kinda wanted to believe their hype ...